1. Field of the Invention
The present invention relates to a digital-to-analog converter (DAC), and more particularly, to a DAC suitably used in an image display device, such as a liquid crystal display or the like.
2. Description of the Related Art
There is a known conventional DAC for a liquid crystal display that has a reference voltage generating circuit, a selection circuit having a plurality of pairs of switches, and a voltage follower. Specifically, when the DAC receives a 6-bit digital signal as an input code, the reference voltage generating circuit, which includes 32 resistors connected in series, supplies 33 different reference voltages to the selection circuit from the terminals of the resistors. The selection circuit includes 37 switch pairs each of which selects one of two inputs in accordance with a corresponding bit of the input code. The selection circuit selects two adjacent reference voltages of the 33 reference voltages when the input code is an odd number, and one reference voltage of the 33 reference voltages redundantly when the input code is an even number, and outputs the selected voltage or voltages to the voltage follower. The voltage follower outputs an average value of two supplied voltages as an analog signal. In other words, when the input code is an odd number, an intermediate voltage between two adjacent reference voltages is generated as a gray-level voltage by the voltage follower (see U.S. Pat. No. 6,373,419).
In this DAC, as the number of input bits (resolution) is increased from 6 to 8 to 10, the number of reference voltages to be generated rapidly increases from 33 to 129 to 513, and the number of switch pairs required for the selection circuit rapidly increases from 37 to 135 to 521. Therefore, the chip size of the DAC is unavoidably increased as the definition level and the number of gray levels of a liquid crystal display is currently increased.
To avoid this, a DAC has been developed in which two adjacent reference voltages are selected and a difference between the two reference voltages is divided by utilizing an ON-resistance of a Metal-Oxide-Semiconductor (MOS) transistor to obtain a desired intermediate gray-level voltage (see US Patent Application Publication No. 2007/0176813).